Shh acts as a mitogen and cell survival factor in many adult processes during normal tissue renewal and in many types of cancer, but acts as a morphogen in several developmental contexts. Whether and how the mitogenic role of Shh is integrated with the morphogenetic role in developmental contexts is still poorly understood. In the limb, Shh regulates both digit number and identity of different digits (A-to-P, thumb to pinky). Shh is thought to act as a morphogen forming a gradient along the limb AP axis, with higher concentrations specifying more posterior digit types. We have determined the time-requirements for Shh function in limb (using a tamoxifen-regulated Cre to remove Shh at different times in mice). To perform this analysis, we generated and characterized a conditional Cre recombinase line selectively expressed in early limb mesoderm, neural crest, gut and tailbud. This line provides an excellent tool available to the scientific community to illuminate different temporal roles of key developmental regulators in several important developmental models using mouse mutants, as well as for genetic lineage tracing studies in mice. As one test of this model, we showed that the Shh activity profiles in different classes of mutants with reduced digit number support the model. In particular, mutants in which central digits are lost first have a reduced duration of Shh activity, but the early transient patterning phase, in which posterior digits are specified, remains intact. Conversely, mutants with selective loss of digit 5 have delayed onset of Shh expression and altered Shh activity during the early patterning phase. To further test this model for Shh function, we have assessed whether restoring survival and/or proliferation in mutant embryos after later-stage Shh deletion can rescue digit formation. To rescue cell survival, the compound mutant for the pro-apoptotic Bcl2 family members Bax/Bak (which play roles in normal interdigital apoptosis) has been introduced to inactivate the intrinsic death pathway. Our preliminary results indicate that both normal digit number and pattern (morphogenesis) can be rescued by simply restoring cell survival and proliferation in Shh mutant embryos. Our results thus far challenge the view that Shh behaves as a classic morphogen and suggest that the roles of Shh during organ morphogenesis and during tumorigenesis may be very similar. Furthermore, our results indicate that there are 2 classes of Shh responsive target genes, those that respond to a transient signal and become stably expressed, and those that require continuous signaling to maintain expression. We are comparing the transcriptomes of Shh mutant and rescued limb buds to characterize the types of genes in these two differentially regulated target classes. In parallel, single cell transcriptome analysis from normal limb buds will be performed to identify expression signatures in the transient Shh signaling phase and characterize immediate-early response zones. Understanding the proliferative and anti-apoptotic actions of Shh in the context of these differentially regulated target classes, will provide a reference for deciphering and intercepting Shh roles in cancer. To learn how proliferation and recruitment of mesenchymal cells relate to the alternating order in which condensations form and integrate our results with other work on Shh function, we are developing tools to image very early condensations, and to trace cell lineage genetically (collaboratively with Dr. Stephen Lockett, NCI). For example, do condensations form via spatially restricted proliferation of precursors in an alternating sequence, or by alternating foci of cell adhesion? We are also using genetic tools including mutants that act downstream of hedgehog signaling to alter digit patterns at early times to learn when and how the alternating sequence of digit formation is determined. These tools will help unravel how the observed alternating anterior-posterior order of digit condensations is regulated, and whether this sequence is evolutionarily conserved among different vertebrates. The possibility of alternating patterns of digit formation may also lead to new insights on the mechanisms of evolutionary digit loss in species with adaptive changes in digit number.